钯-烷/芳基（Pd-R）键溶液态绝对键能的研究

Bond energy is one of the most important factors to determine reaction rate constants, directions and mechanisms, since the essential of chemical reactions is the bond-breaking/making. As to the complexity of the C-H bond activation/functionalization, besides the bond energy of C-H bonds, that of transition metal-alkyl/aryl (M-R) bonds is also a key parameter for the rational design and understanding of transition-metal-mediated organic transformations. Due to the extreme unstability of corresponding intermediates, especially the paucity of the universal and convenient method for bond-energy measurements, there are few efforts focused on the bond energy of M-R bonds (particularly related to the C-H bond activation/functionalization) in past half century, which has seriously hampered the rational development of the field of C-H bond activation/functionalization. Therefore, in this project, the typical palladium complexes (LnXPdR) in C-H bond activation are selected as model compounds. A new method for determining the absolute bond energy of Pd-R bond in acetonitrile solvent is established. And then the parameters such as reaction enthalpies, oxidation-reduction potentials and bond energies of R-H bonds are combined to obtain the homolytic and heterolytic energies of Pd-R bonds and the structure-activity relationship are investigated. Then, the thermodynamic relationship between three cleavage modes of M-R bond was established by thermodynamic cycles, which can be used to evaluate the stability of the corresponding intermediates and analyze the mechanism. The effects of the ligands (such as types, bite angles, electrical resistances, hindrances, etc.) on the Pd-R bond energies and the oxidation-reduction potentials of the intermediates are also examined to explore how structural changes and electron transfer or migration regulate the activity of the catalysts and intermediates. In addition, kinetic studies will be performed as well. This project will provide a theoretical guidance for the rational design of palladium catalysts and the analysis of catalytic mechanisms.

过渡金属-烷基/芳基（M-R）键的键能是理解C-H键活化机理、设计新体系的重要参数。但反应体系复杂、中间体不稳定等因素使得M-R键能相关的研究一直停滞不前，严重阻碍了该领域的理性发展。本项目拟选择对C-H键活化有典型意义的钯的络合物（LnXPdR）作为研究模型，创建测定溶剂态Pd-R键绝对键能的新方法。进而结合量热、氧化还原电位和R-H键键能等，对Pd-R键的均裂能和异裂能进行首次系统测定并研究其构效关系。利用热力学循环建立M-R键三种断裂模式之间的热力学关系平台，用以评价相应中间体的稳定性，为机理分析奠定基础。本项目还将系统考察配体的性质（种类、咬合角度、电性、位阻等）对Pd-R键键能和中间体氧化还原电位的影响，以探究结构变化、电子转移对调控催化剂及中间体活性具有何种规律。除此之外，还拟结合动力学研究，对某些氧化加成反应作案例分析，以期为钯催化剂的理性设计和催化机理的解析等提供理论基础。

钯催化的C-R键转化（R = C, N, O, etc）是构建复杂分子最常用的策略之一，在C-H键官能化、C-R键偶联、烯烃氧化/官能化、C-R键活化等领域得到广泛应用。在这一过程中，钯络合物（M-R物种）作为重要的活性中间体参与到每个基元步骤中，其热力学稳定性（或反应性）作为主要因素决定着整个催化循环的可行性。因此，深入理解钯络合物的基础热力学性质（M-R σ-键的键能）有助于调控反应的热力学驱动力，对优化催化性能（反应效率和选择性）有着至关重要作用。然而，由于缺乏可靠的键能测定方法，过去的几十年间，对M-R键键能（尤其是绝对键能）的相关研究一直停滞不前。项目负责人团队借助其在键能研究中积累的经验，首次建立了溶液相Pd-R键异裂能（ΔGhet(Pd‒R)）的测定方法，并结合热力学循环测得相应的均裂能（BDFE(Pd‒R)）。基于此，首次系统建立了金属-配体（M-L）σ-键的热力学定量标度，用于衡量Pd-R中间体的活性。结果表明，本研究中测定的Pd-R键的异裂能处于2~20 kcal/mol，Pd-R键远弱于相应的R-H键；在极性溶剂中Pd-R络合物像弱布朗斯特酸一样容易发生解离。通过组合使用Pd-R键异裂能（ΔGhet(Pd‒R)）与X-H键异裂能（pKa(R-H)），人们可以从热力学角度分析、乃至预测钯介导的R-H键活化反应中的可行性以及产物的选择性。该成果首次建立了溶液相金属-配体共价键的测定方法，为衡量相关金属中间体的活性提供了定量标度，为新催化剂的设计、新催化反应的开发提供理论指导。